The broad goal of this project is to determine the effects of aging on the cochlea of a new world primate, the common marmoset (Callithrix jacchus). Specifically, this project will determine the functioning of the cochlea and the auditory efferent system by measuring otoacoustic emissions (OAEs). This non-invasive technique, which measures sounds in the ear canal originating in the micromechanical activity of the basilar membrane, provides useful information on the health and status of the cochlea, particularly the outer hair cells (OHCs). When there is cochlear damage or hearing loss, OAEs are absent or their strength is diminished. The efferent system improves hearing in noise by inhibiting OHCs and thereby attenuates OAE amplitudes. It can be activated by stimulating the contralateral ear. When the efferent system is ineffective OAEs are not readily suppressed by contralateral stimulation. OAEs have important clinical and diagnostic applications in addition to providing information on certain cochlear processes, and thus they are useful for determining age-related changes in cochlear functioning. The marmoset is an attractive model system for this study because it is a relatively short-lived primate, having an average life span of seven to eight years, is easily bred in captivity, and relies on vocal communication for maintaining contact with group members. While the marmoset is emerging as a model for studying central auditory function, there are no studies on the auditory periphery of this species. This project will therefore take the first steps towards determining the status of the marmoset cochlea and provide data on the effects of aging on cochlear functioning. The experimental goals of the project are to measure evoked OAES (i.e. emissions that are elicited in response to acoustic stimulation of the tympanic membrane). Two types of evoked emissions will be measured, namely distortion product (DP) OAEs and click-evoked (CE) OAEs, so-called because the former uses two pure tones to elicit a (nonlinear) cubic distortion product, while the latter uses an impulsive click to elicit broadband responses. The experimental goals of the project will (1) determine the effects of aging on measured OAE responses, and (2) determine the functioning of the auditory efferent system by measuring OAEs in response to stimulation of the contralateral ear. The long-term goal of the research is to comprehensively investigate the marmoset auditory periphery using a variety of non-invasive and electrophysiological procedures, and develop prosthetic implants for treating hearing loss. The project will determine the effects of aging on the functioning of the cochlea (the hearing organ) of a new world primate, the common marmoset (Callithrix jacchus). This study will measure otoacoustic emissions (OAEs), which are sounds reflected back from the cochlea in response to acoustic stimulation of the ear. OAEs provide information on the health of the cochlear amplifier, a physiological process that is important for normal hearing, and thus it can be used to determine the effects of aging on cochlear functioning.